While learning for an exam, I stumbled over the following Question:

According to Material Science of thin Films by Milton Ohring,

"RF sputtering essentially works because the target self-biases to a negative potential. Once this happens, it behaves like a DC target where positive ion-bombardment sputters away atoms for subsequent deposition."

So far so good. What I don't understand is

  • why exactly a self-bias voltage is appearing?
  • why this bias voltage does not lead sputter bombarding of the substrate, that should be coated?

I understand that it relates to the high mobility of electrons compared to the ions, but I don't see why this implicates the self-bias yet. The above mentioned source tries to explain it as follows:

"Negative target bias is a consequence of the fact that electrons are considerably more mobile than ions and have little difficulty in following the periodic change in the electric field. The disparity in electron and ion mobilities means that isolated positively charged electrodes draw more electron current than comparably isolated negatively charged electrodes draw positive ion current. For this reason the discharge current-voltage characteristics are asymmetric and resemble those of a leaky rectifier or diode [...]"

This is not clear to me, as I would think that this explanation would hold for the sputter substrate as well. I would be very glad if someone could make that clear by a somehow intuitive explanation. Thank you for all suggestions. :)


The answer to your first question has already been given by your source (mobility difference of electrons and ions).
The answer to the second question has two components: 1)The source of the sputtering is on top of the substrate, 2)Since the top surface is negatively charged, the bottom of the substrate is positively charged, which repels the few stray positive ions that might manage to get below.

EDIT: Because of the comment below I will add an example that I hope makes it clear how the self-bias comes about.
On the left side you have "sprinters" and on the right side you have "slow pokes." Each one can carry one bucket of water and dump it into a barrel. It should be obvious, that after some time, the left side will have more water in their barrel than the right side. Thus, a water difference is created! With reference to electrons, more electrons on one electrode/side, creates a voltage difference between the electrodes.

  • $\begingroup$ OP is specifically saying that they don't understand why mobility difference leads to negative self-bias (and, indeed, mobility difference by itself is insufficient to explain this). $\endgroup$ – Al Nejati Oct 3 '18 at 22:38
  • $\begingroup$ Your analogy of water buckets is nice, and indeed resembles how I would think about this. What I still wonder if this also applies to conducting materials as a target material (because they would not be isolated and thus don't charge up? well, maybe they are decoupled by a capacitor or something.. I guess with this followup question it might get too technical for a qualitative explanation). $\endgroup$ – Pidrittel Oct 9 '18 at 9:00
  • $\begingroup$ Regarding the second question, when you say The source of the sputtering is on top of the substrate, you want to imply by that that the target material "rains down" on the substrate, but not the other way around, because the substrate material will not fall up? The second part of your answer is unclear though, because it seemst that this argument would apply to the sputter target as well (which it can not, obviousely). So I still don't get why the sputter target is bombarded with ions while the substrate is not. $\endgroup$ – Pidrittel Oct 9 '18 at 9:01

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